The present invention relates in general to controlling the frequency response of an acoustical system and more particularly concerns novel apparatus and techniques for improving the acoustical frequency response of a system having spaced loudspeakers operating over a common frequency range. The invention is especially useful in reducing undesired peaks and dips in an automotive sound system having speakers installed in different locations.
In an automotive sound system, speakers are often installed in different locations. If the speakers radiate sound in a common bass frequency range, several problems may arise. Cancellation or reinforcement respectively of signals at the same frequency coming from two or more spaced loudspeakers may produce dips and peaks in the frequency response at the listener's location. These dips and peaks are particularly objectionable in music reproduction at the lower frequencies for which the wavelength is large compared to the diameter of the listener's head.
A typical prior art approach uses minimum-phase networks for equalization of multi-speaker systems. However, minimum-phase networks cannot sufficiently smooth the low frequency response at the listener's position because of the inherent constraint between the phase and magnitude of the frequency response of minimum-phase networks. The problem is more difficult to solve for multiple listener locations, for example, in front and rear seats of an automobile. Solving the smoothing problem then requires more degrees of freedom in design than achieving equalization for a single location.
It is an important object of this invention to provide improved acoustical response in a region energized by multiple spaced loudspeakers.